A common application for a two-speed direct current motor is as the drive motor for driving the windshield wipers of a motor vehicle at either a low speed or a high speed. Typically, the driver of the motor vehicle positions a wiper switch at either a low speed setting or a high speed setting whereby electric power is directed through associated electronic circuitry to power the direct current motor. One type of commonly used two-speed direct current motor has three brushes, comprising a low speed brush, a high speed brush and a common ground brush, for supplying current to the windings on the armature of the motor. Each of the brushes contacts a commutator located on and rotating with the armature shaft. A pair of the brushes, generally the low speed brush and the common ground brush, are disposed in circumferentially spaced relationship substantially 180.degree. apart. In operation of the motor, current is passed from a power source through the low speed brush to the commutator, thence through the motor windings and back to the power source through the common ground brush positioned substantially 180.degree. circumferentially from the low speed brush.
When current is supplied to the low speed brush, the motor operates at a first lower rotational speed (lower RPM) and the wipers sweep at a lower speed, for example 45 cycles per minute. When it is desired to operate the motor at a second higher speed, current is instead supplied to the windings through the high speed brush which is positioned in circumferentially spaced relationship with and between the low speed and the common ground brushes. As is well understood by those skilled in the art, when the current is supplied to the motor through this third brush, which is circumferentially closer to the common ground brush than the low speed brush is, the rotational speed of the motor is increased to a second higher RPM (revolutions per minute) and the wipers sweep at a higher speed, for example 65 cycles per minute. The relative difference between the lower rotational speed and the higher rotational speed of the motor is determined by the offset, i.e. the angle of separation, of the high speed brush from the common ground brush.
An inherent problem experienced in operation of two-speed direct current motors arises from electrical wear, commonly termed erosion, of the end face of the high speed brush. This erosion is caused by the electrical arcing which occurs as contact between the brush end face and the commutator is broken. Due to the positioning of the high speed brush such that commutation with the armature windings occurs when the windings are passing outside of the neutral zone of the magnetic fields established by the field magnets of the motor, which may be formed by permanent magnet poles or wound electromagnet poles, the arcing is more severe than experienced at the low speed and common ground brushes which are positioned such that commutation with the armature windings takes place when the windings are passing within the neutral zone of the magnetic fields established by the field magnets.
The erosion of the end face of the high speed brush results in a shift of the effective contact line between the brush and the commutator as the high speed brush seats on the commutator during the early phase of its operational life, typically over the first couple hundred or so hours of operation of the motor. Thus, the effective contact line between the high speed brush and the commutator shifts towards the leading edge of the brush away from its initial position at the center of the brush, which is where the contact line was positioned when the motor was operated for specification testing prior to being placed into service. This shift in the effective line of contact adversely impacts motor performance as it results in an increase in the RPM of the motor at high speed operation beyond the desired RPM of the motor at high speed operation, causing the wipers to sweep at up to 75 to 80 cycles per minute rather than the desired 65 cycles per minute at the higher speed setting. For a worn-in motor, the actual high speed RPM may be well above the high RPM measured during specification testing and actually be outside of the acceptable range of specification speeds.